1. Field of the Invention
The invention relates to data processing systems and more particularly to a method and apparatus for communicating among a plurality of network nodes connected to the hub of a local area network.
2. Description of the Related Art
A Local Area Network, or LAN, is a data communications system which allows a number of independent devices to communicate with each other within a moderately sized geographical area. The term LAN is used to describe networks in which most of the processing tasks are performed by a workstation such as a personal computer rather than by shared resources.
A LAN node consists of a desktop workstation which performs processing tasks and serves as the user's interface to the network. A wiring system connects the workstations together, and a software operating system handles the execution of tasks on the network.
The configuration of the various pieces of the network is referred to as the topology. In the star topology the switching control is at the center of the network, with all of the attached devices, the individual workstations, shared peripherals, and storage devices, on individual links directly connected to the center of the star. In the star configuration, all of these devices communicate with each other through the center which receives signals and transmits them out to their appropriate destinations.
The ring bus topology employs a circular bus route known as a ring. In a slotted ring configuration, signals pass around the ring to which the devices are attached.
Ring networks are flexible in that new devices can be easily added and taken away. But because the signal is passed from end to end on the ring bus, the length of the network cable is limited. Star topologies have the advantage that the workstations can be placed at a considerable distance from the center of the star. A drawback is that star topologies tend to be much slower than bus topologies because the center of the star is involved in every transmission.
In a star configuration, the signaling method is different than in bus or ring configurations. In the star configuration the center of the star is involved in all of the communication signals. In a bus topology, communications messages have no central controller. Each device attempts to send signals and enter onto the bus when it needs to. If some other device tries to enter at the same time, contention occurs. To avoid interference between two competing signals, bus networks have signaling protocols that allow access to the bus by only one device at a time. The more traffic a network has, the more likely it is that a contention will occur. Consequently, the performance of a bus network is degraded if it is overloaded.
Ring bus configurations have more complex signaling protocols. The most widely accepted method in ring networks is known as the token ring, a standard used by IBM. An electronic signal, called a token, is passed around the circuit collecting and giving out message signals to the addressed devices on the ring. There is no contention between devices because a device does not signal to gain access to the ring, it waits to be polled by the token. The advantage is that heavy traffic does not slow down the network. However, it is possible that the token can be lost or it may become incoherent or disabled by failure of a device on the network to pass the token on.
In the above-identified copending application Ser. No. 07/291,700 there is described a local area network architecture based on a active star topology. Nodes attach to the hub of the star through duplex communication links. Messages transferred between nodes are passed through the hub, which is responsible for arbitration and routing of messages. Each node of the active star responds only to those messages that are addressed to it. Routing of messages is accomplished by a destination address in the header of the message. These addresses are unique to each node and provide the means by which the hub keeps the communication between nodes independent.
It is an object of the present invention to provide a hub for a star local area network which provides a means for routing messages received at the hub without the need for special arbitration mechanisms.